1. Field of the Invention
The present invention relates to baseband signal demodulators in a receiver operated on a time division multiple access (TDMA) basis, and more particularly, to a baseband signal demodulator in a digital radio communication system capable of performing easy and stable processing of a plurality of data for a plurality of time slots including the demodulation of a baseband signal.
2. Description of the Related Art
To reduce the size and the power consumption of a mobile device and a base station in a radio communication system, the modem section of the mobile device and the base station performs modulation and demodulation of data in a digital signal processing manner in which waveform shaping, etc. is performed.
In a digital radio communication system based on TDMA communication, the modem section of the base station receives baseband signals from a plurality of mobile devices on a time-division time slot basis. Since the distance between the base station and the mobile devices and performance of the mobile devices differ for each mobile device, the optimum sampling points of baseband signals received by the base station from the respective mobile devices differ from each other for the respective time slots.
To perform digital signal processing on the baseband signals having phase errors between time slots, it is necessary to subject the baseband signals to a sampling operation at a constant sampling interval and then to an analog-to-digital (A/D) converting operation to obtain digital signals for subsequent operations.
However, since the received baseband signals have phase errors between time slots, the optimum sampling points (time at which the eye of the pattern of the received baseband signal is opened) for the A/D conversion of the received baseband signals are different from each other for the respective time slots. For the purpose of reducing a data transmission error during digital signal processing, a receive clock obtained from the received baseband signal is used for the digital signal processing.
For this reason, when the receive data sampled through the A/D conversion is subjected to digital signal processing such as waveform shaping and detection using a digital filter, it is necessary to perform the digital signal processing using a sampling clock synchronized with the receive data for each time slot as a processing clock, with the processing clock being required to be changed for each of the time slots.
However, when the digital signal processing of data for one time slot is not completed within the duration of this time slot, the processing clock must be effective during a period exceeding this time slot, which arises a problem that the handling of the processing clock is complicated.
In order to avoid the above problem, it may be so constructed that the received baseband signal is subjected to a sampling operation using a system clock of a timing common to the respective time slots and signal processing is performed using the system clock as a signal processing clock. However, this causes another problem that sampling of the received baseband signal cannot be made at the optimum sampling point, which leads to an increase in the data transmission error.
For this reason, it is impossible to use as a clock for processing of digital signals the system clock which is commonly used in the system except for the radio section and the modem sections of the base station.
Further, in the modem section, since the digital signal processing is not carried out using the system clock, there arises still another problem that a delay means is required for synchronizing a demodulation signal for each of the time slots with the system clock in subsequent demultiplexing operations.